The present invention relates to image processing systems and methods and more particularly to systems and methods for scanning halftone images to make quality copies or prints.
Various kinds of machines or systems employ a scanner to obtain an image of a document for further processing leading to reproduction, copying, or printing of the document. For example, copy machines employ a platen on which a document is placed for scanning, with the scanned electronic image being processed by the machine to produce one or more copies. Similarly, a FAX machine employs a scanner to obtain an image of an inserted document, with the scanned electronic image being processed by the machine to generate a local or remote copy. A printer system may also employ a separate scanner to obtain an electronic image of a document to be printed, and the electronic image may then be processed by a separate computer system to generate an image in a form ready for printing by the system printer.
In the prior art, the scanning and subsequent processing of black-and-white and color halftone or binary images, such as those found in newspapers and magazines, has typically resulted in undesirable loss of detail in the output copies or prints. Normally, such loss of image detail has resulted in part from an inability of the total system including the scanner to respond to relatively high spatial gradients across edges contained in the image. Loss of image detail in the prior art has also resulted from an inability of entire systems to compensate for variation in printing density or paper density across the original image, such as faded inks, stains, rubbed areas, etc.
With respect to high spatial gradients in images to be reproduced, the conventional scanner typically has a resolution comparable to the resolution with which the image to be reproduced was created. The scanner is thus enabled to read accurately image areas having low and medium spatial gradients, but it is limited in reading areas such as edges having high spatial gradients.
For example, edge areas are typically read such that gray pixels result at the borders between black and white areas in a black and white image. Thus, the aperture of the scanner bridges multiple image pixels which are value averaged, and, at the edges, the averaging of black and white pixels results in gray pixels. The end effect is that image detail is lost.
The following prior art has limited relevance to the present invention:
1. U.S. Pat. No. 4,926,267, entitled "Reproduction of Halftone Original With Reduced Moire," dated May 15, 1990, and issued to J. Shu, et al.
2. U.S. Pat. No. 4,977,605, entitled "Binary Quantification of an Image Having Multiple Levels of Greys", dated Dec. 11, 1990, and issued to Fardeau, et al.
3. U.S. Pat. No. 4,468,705, entitled "Data Transition Enhancement", dated Aug. 28, 1984, and issued to Jack D. Burton.
4. U.S. Pat. No. 4,876,610, entitled "Image Processing Apparatus With Binarization-Error Dispersal", dated Oct. 24, 1989, and issued to H. Ohsawa et al.
5. U.S. Pat. No. 2,892,887, entitled "Apparatus For Producing Screened Printing Forms With Automatic Correction of Tone Values", dated Jun. 30, 1959, and issued to Rudolf Hell.
6. U.S. Pat. No. 4,554,593, entitled "Universal Thresholder/Discriminator", dated Nov. 19, 1985, and issued to Sidney J. Fox, et al.
7. U.S. Pat. No. 4,259,694, entitled "Electronic Rescreen Technique for Halftone Picture", dated Mar. 31, 1981, and issued to Henry H. Liao.
8. U.S. Pat. No. 4,638,369, entitled "Edge Extraction Technique", dated Jan. 20, 1987, and issued to Robert C. Hsieh.
9. U.S. Pat. No. 2,962,550, entitled "Resolution Restorer System", dated Nov. 29, 1960, and issued to Robert M. Brink.
10. U.S. Pat. No. 3,249,690, entitled "Video Quantizer Producing Binary Output Signals at Inflection Points of Input Signal", dated May 3, 1966, and issued to Ernest J. Schubert.
11. U.S. Pat. No. 4,633,327, entitled "Enhancement Halftoning", dated Dec. 30, 1986, and issued to Paul G. Roetling.
12. U.S. Pat. No. 4,730,221, entitled "Screening Techniques by Identification of Constant Grey Components", dated Mar. 8, 1988, and issued to Paul G. Roetling.